1. Field of the Invention
The present invention relates broadly to the field of biomedical transducers and more particularly to a passive sensor for fetal monitoring.
2. Related Art
Traditional methods of fetal monitoring include palpation and stethoscopic examination. These methods rely heavily on the treating physician's ability to receive sensing information and to subsequently process that information. Hence, such methods are inherently qualitative rather than quantitative.
Ultrasonic techniques improve upon traditional methods, but are intrusive and have possible deleterious effects on fetal development. Ultrasonic equipment is also costly, ill-suited to portability because of its size, and subject to error arising from imprecision in sensor alignment.
The prior art of fetal sensors includes acoustic sensors, which detect acoustic signals originating within the abdomen of the mother, interference sensors which detect and cancel interference from the mother's heartbeat, and accelerometers which detect fetal movement.
Prior interference sensors, such as those described in U.S. Pat. No. 4,784,154 (Shirley) have utilized a split polarization arrangement of the sensor assembly to cancel common mode signals arising from electromagnetic interference and radial stresses. Such an arrangement precludes the detection of fetal movement, because fetal movement generates common mode signals.
An overview of prior art monitors reveals two basic limitations: (1) they respond to fetal heart rate, thereby forfeiting the vast store of potential information contained in an instantaneous time history of pressure pulses; and (2) they are ill-suited to an ambulatory mode of operation, requiring operation in a clinical setting.
The monitor disclosed in U.S. Pat. No. 4,781,200 (Baker) surmounts these limitations. The described embodiments of Baker, however, reveal other difficulties. First, the high surface mass density and cantilevered arrangement of piezoelectric crystals used for signal detection make the crystals highly sensitive to movement by the mother. Consequently, the monitor of Baker, which consists of acoustic and interference sensors, together with a signal processing algorithm, must extract a relatively weak fetal signal from strong interference due to movement by the mother. Another limitation of Baker is the bulkiness of the cantilevered piezoelectric crystals which makes the belt on which they are mounted thick, cumbersome, unattractive, and generally impractical. Moreover, the monitor does not avail itself of array signal processing techniques, whereby several sensors operating in concert yield substantially more information than a sensor operating alone. Finally, in the mounting arrangement of Baker the sensors are not on a common plate, causing the sensors to yield different responses to a common mode signal.
Accordingly, it is an object of the invention to provide an ambulatory, passive sensor suitable for fetal monitoring.
It is a further object of the invention to provide a sensor suitable for use in a fetal heart monitoring system.
A further object of the invention is to provide an ambulatory, passive sensor exhibiting an acoustic impedance closely matching that of a human torso.
A further object of the invention is to receive pressure pulses emitted by the fetal heart and transform these pressure pulses into an electrical form suitable for processing by a signal processing means.